Re: Need suggestion on compensating clock drift on Xen based VPS

Ryan Malayter wrote:
On Sat, Aug 22, 2009 at 3:28 PM, Aryanto Rachmad<aryanto@xxxxxxxxx> wrote:


I know that it is problematic to run ntpd on Xen domU, but could anybody
give me some suggestion to have a stable system clock?


So, in virtual machines, "time" is variable. Your VM gets de-scheduled
frequently, for tenths of a second at a time. Which makes timekeeping
pretty tough. Even acurate CPU utlization is near-impossible to
measure from within a guest VM.

The usual "solution" is to have the hypervisor (dom0 in the Xen case)
run ntpd, and provide time to the guest VMs using a vendor-specific
driver. For VMware, this driver bundle is called "VMware Tools". For
the commercial Citrix XenServer, it's called "Xen Tools". For your
VPS, you should talk to your hosting provider I suppose.

In general though, virtualization is not very compatible with good
time keeping. But you should be able to do better than
three-second jumps.We generally see <100 ms offsets in guests on a
very busy VMware ESX 3.5u4 cluster.

See also:
http://support.ntp.org/bin/view/Support/VMWareNTP

The VMware whitepaper listed there goes deep into the problem of
providing good time to guest operating systems.

Only someone who was trying to sell VM systems would even bother - its
way to simple to maintain the VM system as a client and make all of this
noise in the wind.

Todd Glassey (as an Auditor).

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